。
She described the incident with vivid, lively prose.
Literal
She [topic-は] that case [object-を] vividly-doing brushstrokes [with-で] described.
生き生きとした is a complex modifier: 生き生きと (a 擬態語-style adverb 'vividly, lively,' from doubled 生き 'alive') + した (past form of する acting as adjectival 'made/that') — together: 'lively-and-made,' attributive form for 'vivid.' This ~とした pattern derives many adjectives from mimetic adverbs: ぼんやりとした ('vague'), はっきりとした ('clear'), しっかりとした ('firm'). 筆致 ('brushstrokes / writing style') is a literary noun extending the calligraphic metaphor of the 'brush' to writing in general. 描写する ('depict, describe') is itself drawn from visual art — verbal painting.